This invention relates to audio systems and, more particularly, to an apparatus and method for decoding four individual audio signals contained in two composite signals, the decoding achieving an improved degree of quadraphonic realism when the decoded outputs are reproduced.
In the U.S. Pat. No. 3,708,631 there is the a sound system wherein four individual audio signals, designated L.sub.f, L.sub.b, R.sub.b and R.sub.f are encoded in accordance with the "SQ" quadraphonic technique to produce two composite signals designated L.sub.T and R.sub.T. These two composite signals are typically transmitted over two lines, or recorded on and reproduced from two channel recording medium, such as a stereophonic disc record, and subsequently decoded into four simulated channels of sound by suitable decoding apparatus, a form of which is described in the referenced patent. In the SQ quadraphonic system, the composite signal L.sub.T contains, to thhe extent they are present, L.sub.f in a dominant proportion and L.sub.b and R.sub.b in subdominant proportions, L.sub.b and R.sub.b being phase shifted with respect to each other. Also, R.sub.T contains, to the extent they are present, R.sub.f in a dominant proportion and L.sub.b and R.sub.b in subdominant proportions, L.sub.b and R.sub.b being phase shifted with respect to each other. The referenced patent demonstrates that L.sub.T and R.sub.T can be decoded using an SQ decoder matrix to produce four signals which may be designated L.sub.f ', L.sub.b ', R.sub.b ' and R.sub.f ', each of these signals containing, in predominant proportion, a corresponding one of the four individual signals, along with certain "unwanted" components in subdominant proportions. These decoded signals are not "pure" or discrete original signals, each being "diluted" by two other signals. Nevertheless, when all four channels of the original program contain musical signals in concert, and the four decoded signals are reproduced over respective loudspeakers which are, for example, placed in the corners of a room or a listening area, then as far as the listener is concerned there is sufficient "mixing" of the sounds in the room that the resulting overall sound effect is quite similar to the sound of the original four discrete channels, and a credible simulation of the original four channel program results.
There are situations, however, in which it is desirable to provide an illusion of greater independence or purity of the decoded signals; for example, when the original sound is present in one or two channels only, it is desirable to enhance the separation of the channels which are present. Systems for achieving such audible spatial enhancement are described, for example, in the above-referenced U.S. Pat. No. 3,708,631 and the U.S. Pat. No. 3,784,744. It is an object of the present invention to obtain greater quadraphonic realism than that obtainable using previously described techniques.